Coupling circuit



Mmm WS. um@ HQFORBES A www@ COUPLING CIRCUIT Filed May 18, 1932 {n/HY C WHBE@ Patented Apr. 7, 1936 UNITED STATES COUPLING CIRCUIT Henry C. Forbes, Anderson, Ind., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application May 18, 1932, Serial No. 612,117

14 Claims.

This invention relates to a coupling circuit for a vacuum tube amplifier or similar device; and more particularly to a coupling circuit having good gain characteristics and flexibility of design and use.

An object of this invention is to provide a coupling circuit for a signal translating device which will provide a substantial gain over the entire frequency range of signals to be received. This has been accomplished by providing a plurality of cooperating tunable resonant circuits.

Another object of this invention is to provide a circuit for coupling vacuum tubes which has flexibility of design as well as desirable operating characteristics. This has been accomplished by providing tuning means for each of the coupled circuits.

Another object of this invention is to provide a coupling circuit for a radio frequency amplifier with more nearly ilat gain characteristics. than the conventional circuits in use at the present time, or gain characteristics which are complementary to the characteristics of the rest of the receiver. This has been accomplished by tuning the output circuit of one tube as well as the input circuit of another tube.

Another object of this invention is to provide a circuit which while having any or all of the above characteristics may be controlled by a single tuning control. This may be accomplished by actuating the tuning of all of the tunable units from a single control.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein a preferred embodiment of one form of the present invention is clearly shown.

In the drawing:

Fig. 1 discloses a form of coupling circuit involving the instant invention.

Fig. 2 discloses a gain curve for a coupling circuit which .does not have a plate choke, or which has a plate choke that has a resonant F frequency that is so far out of the band of signals to be received that no advantage is gained from it.

Fig. 3 shows curves for a coupling circuit having -a choke Xedly tuned to resonance at a frequency below, but not much below the band of frequencies to be received.

Figs. 4 and 5 show curves for a coupling circuit such as that disclosed in Fig. l.

Fig. 6 discloses a modification in the type of coupling used.

In Fig. 1 in particular, an anode or output electrode I4 of a thermionic tube It is connected to one end of an inductance or choke I2. The other end of inductance I2 is provided for connection to a suitable source of DI. C. potential through a lead It. A v-ariable condenser I8 is connected across the inductance I2 to form a tunableV resonant circuit 2li. A control electrode or grid 2`and a cathode 2B of the thermionic tube I0 are adapted for connection to the output of a similar and preceding coupling device, or any Idesired circuit through leads 2d and 28, respectively. A control or input electrode 32 of a thermionic tube 3Il is connected to one end of an inductance 34. The other end of the inductance 34 is connected to a cathode 36 of the therlnonic tube 3%. A Variable condenser 38 is connected across the inductance 34 to form a tunable resonant circuit til. A condenser 42 is connected to theV anodev I4` of the thermionic tube I0 and to the control electrode 32 of the thermionic tube 3l). An anode or output electrode 44 of the thermionic tube 30 is adapted for connection with a siinilar coupling circuit, or` any other suitable circuit through a lead 46. The variable con'- densers I8` and 38 are indicated as being controlled from a common control element 50.

With particular reference to Fig. 2, a curveV Ilt is plotted with frequency as the abscissa and gain asthe ordinate. The frequency limits of the curve indicated by the vertical line at 550 and i566 indicate the extremities of the frequency band to be received.

With particular reference to Fig. 3 a curve 200 and a curve 220 are plotted with frequency as the abscissa and voltage as the ordinate; The curve 2B@ represents the voltage developed across a'tuned-grid lcircuit of a radio frequency stage which has an inductance therein having a resonant frequency somewhat above the highest frequency to be received. The curve 220 represents the voltage developed across a xed tuned plate choke in the radio frequency stage. A curve 2 I0 is plotted with frequency as the abscissa and gain as the ordinate. This curve represents the gain of? a radio frequency stage having the grid circuit and choke characteristics indicated by curves 20|] and 229.- The frequency limits of the curves 2I0 and -20'llas indicated by the vertical lines at 55D and |5011 represent the limits of the band'of frequencies to be received.

With particular reference to Fig. 4, a curve 30!!V and a curve 320 are plotted with frequency as the abscissa and voltage as the ordinate. The curve 300 represents the voltage developed across the resonant circuit 40 in Fig. 1 as that circuit is tuned to resonance at diierent frequencies. The curve 320 represents a resonance curve of the resonant circuit 20 of Fig. 1 for a predetermined setting of the condenser I8 of Fig. 1. The curve 3I0 represents a gain curve for the circuit of a single stage as shown in Fig. 1 when the resonance curve of the resonant circuit 2l] is placed as indicated by the curve 320 and the tuning of the resonant circuit 40 is varied. Gain for the circuit in Fig. 1 is represented by the ratio of EzzEi. The frequency limits as indicated by the vertical line at 550 and |500 indicate the limits of the band of frequencies to be received. The vertical line at A indicates a desirable working frequency, or frequency of a signal to be received 'when the resonant circuit 20 is tuned to a frequency as indicated by the curve 320, and the resonant circuit 4D is tuned to the frequency indicated by the intersection of line A with the curve 3GB. For those conditions, the gain is indicated by the ordinate of a point B.

With particular reference to Fig. the curves represented by numbers similar to those in Fig. 4 represent similar curves. In this figure, however, the resonant frequency of the resonant circuit 20 has been changed to a higher frequency value. The line AA represents the working frequency, or a desirable frequency to be received, for the conditions represented. The gain in this case is represented by the ordinate of a point BB.

With particular reference to Fig. 6, a primary inductance 50 has a variable condenser 62 connected thereacross to form a resonant circuit 64. The primary inductance G has leads 66 and 68 connected to its extremities for connection to the output of a preceding signal translating device. A secondary inductance 'l0 has a variable condenser 12 connected to its extremities to form a resonant circuit 14. The inductances 60 and l0 are magnetically coupled, and hence, a mutual inductance M exists between them. The extremities of the inductance are connected to a control electrode 16 and a cathode 18 of a thermionic tube 8G to provide signal output means for the circuit. The measurement of gain for a circuit of this type would be made in the same manner as for the circuit shown in Fig. 1.

It will be apparent to one skilled in the art that any increase in the voltage developed across the plate choke or inductance I 2 will increase the gain of the stage. It is therefore apparent, that if the resonant frequency of the inductance l2 is varied by tuning it with a condenser so that its resonant frequency is always near that of the signal frequency of the signal being received, the gain may be increased over what it would be if the inductance were tuned to a fixed frequency so far outside the band of signals to be received that an appreciable voltage rise would not be obtained. If the inductance is tuned to a fixed frequency below, but not much below, the band of signals to be received as indicated in Fig. 3 the voltage rise across that inductance will be apparent at the low frequency end of the band but will not appreciably aid the gain at the higher frequencies. Hence, by tuning the resonant circuits 20 and 40 in unison so that the resonant frequency of the resonant circuit 2D remains somewhat below the frequency of the signal being received, a beneficial gain may be derived for any frequency in the band.

If a fiat gain curve is desirable the benefit derived from the tuning of the resonant circuit 2i) may be varied as required by the rest of the circuit by so shaping the plates of the condenser i8 that they tune the resonant circuit 20 to a desired frequency difference from the frequency of the signal being received. That is, the closer the resonant frequency of the resonant circuit 20 comes to the frequency of the signal being received the higher the voltage that will be developed across the resonant circuit 20 and consequently, the higher the gain will be. Thus, by shaping the plates of the condenser I8 the frequency difference between the resonant frequency ao'sasct of the resonant circuit 20 and the frequency of the signal being received may be kept at an amount necessary to produce the flat gain char; acteristic.

In like manner, if it is desirable to have a gain curve slope in order that it may be complementary to the gain curves of other stages of the receiver or device or to compensate for attenuation in the circuit with which it is used, the plates of the condenser l may be so shaped that the effect of the resonant circuit 20 will produce the desired result.

By comparison of the curves in Figs. 2 and 3 it may be seen that even by leaving the choke tuned at a fixed frequency increases the average gain over the band of signals to be received. Then by comparison of the curves in Figs. 3, 4 and 5 it may be seen that by tuning the resonant circuit 2 a greater gain may be obtained at some points of the band as well as a more uniform gain.

Reasoning similar to that applied to Fig. 1 may also be applied to Fig. 6. That is, the higher the voltage that is developed between the extremities of the primary inductance 60, the higher will be the voltage induced in the inductance 70. Hence, since by tuning the primary inductance to a frequency at or near the frequency of the signal being received, the voltage developed across the inductance will be higher than it would be if the inductance were not tuned and had a resonant frequency farther from the value of the received signal frequency. In view of the preceding discussion, it is quite apparent, then that the gain characteristics of a magnetically coupled circuit may be improved and regulated by tuning the primary inductance along with the secondary inductance.

While the form of embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

l. A circuit for coupling the output of one vacuum tube to the input of another vacuum tube, including in combination, an output electrode in the first mentioned vacuum tube, an inductance connected to said output electrode, a variable capacity for changing the resonant frequency of said inductance, an input electrode in the second vacuum tube, a second inductance connected to said input electrode, a variable capacity for tuning said second inductance to resonance at a predetermined frequency, the resonant frequencies of said inductances being different at all times, means capacitively coupling said inductances, and a single means for actuating both of said variable condensers together, the resonant frequency of the rst inductance at any setting of the signal means being close to, but below, the frequency of the second inductance.

2. A circuit for coupling the output electrode of one vacuum tube to an input electrode of another vacuum tube, including in combination, an inductance 'connected to said output electrode, variable means for changing the resonant frequency of said inductance, a second inductance connected to said input electrode, variable means for changing the resonant frequency of said second inductance, the resonant frequencies of said inductances being different at all times, and means for capacitively coupling said inductances, the resonant frequency of the first inductance being constantly below, but close to, the resonant frequency of the second inductance.

3. A circuit for coupling the output electrode of one vacuum tube to an input electrode of another vacuum tube, including in combination, an inductance connected to said output electrode, means for tuning said inductance to change its resonant frequency, a second inductance connected to said input electrode, means for tuning said second inductance to change its resonant frequency, the resonant frequencies of said inductances being different at all times, and means for coupling said resonant circuits, the resonant frequency of the first inductance being constantly below, but close to, the resonant frequency ofi the second inductance.

4. A circuit for coupling the output electrode of one vacuum tube to an input electrode of another vacuum tube, including in combination, a tunable resonant circuit connected to said output electrode, a tunable resonant circuit connected to said input electrode, the resonant frequencies of both circuits being different, means for coupling said resonant circuits, and means for tuning said resonant circuits in unison, the resonant frequency of the first circuit being below, but close to, the resonant frequency of the second circuit at all times.

5. In a radio frequency amplifier, the cornbination comprising, a thermionic tube having an anode therein, a second thermionic tube having a control electrode therein, a choke connected to said anode, a variable condenser connected across said choke to vary the resonant frequency of the anode circuit, an inductance connected to said control electrode, a second variable condenser connected across said inductance to vary the resonant frequency of the control electrode circuit, the resonant frequencies of both circuits being different, a condenser connected to said choke and said inductance to furnish coupling between them, and means f or varying both of said variable condensers in unison, the resonant frequency of the choke being below, but close to, the resonant frequency of the inductance at all times.

6. In a radio frequency amplifier, the combination including, a thermionic tube having an output electrode, another tube having an input electrode, a tunable resonant circuit connected to said output electrode, a second tunable resonant circuit connected to said input electrode, the first mentioned tunable resonant circuit always being tuned to a frequency below, but not much below the frequency to which said second resonant circuit is tuned.

7. In a coupling circuit for a radio frequency amplifier, the combination including, a tunable resonant circuit, a second tunable resonant circuit, means for coupling said circuits, and unicontrol means for tuning said circuits so that the .first mentioned tunable resonant circuit is always tuned to a frequency below, but not much below the frequency to which said second resonant circuit is tuned.

8. A signal translating circuit for the reception of signals within a predetermined band of frequencies, including in combination, an inductance having a natural resonant frequency above the high frequency end of said band, a variable condenser for tuning said inductance to the frequency of the signals received, a tunable resonant circuit including an adjustable tuning means capacitively coupled with said inductance, uni-control means for varying the variable condenser and the tuning means, said tunable resonant circuit being tuned at all times to a frequency sufficiently below, but close to, the signal frequency to obtain a substantial voltage gain across said translating circuit for the signal frequency being received.

9. A signal translating circuit for receiving signals within a predetermined frequency band, including in combination, a tunable resonant circuit tuned to resonance at the frequency of the signals being received, a second tunable resonant circuit tuned at all times to resonance below, but close to the frequency of the signals being received, means for coupling said resonant circuits to permit signal energy transfer between them, and means for tuning said resonant circuits in unison.

10. In a coupling circuit for a signal translating device, the combination including a plurality of coupled resonant circuits, and means for tuning said circuits in unison, one of said circuits being resonant at the signal frequency, and another of said circuits being constantly resonant below, but close to the signal frequency.

11. In a coupling circuit for a signal translating device, the combination including, a plurality of coupled resonant circuits, and means for tuning said circuits in unison over a band of frequencies, one of said circuits having its tuning varied so that it is resonant at the frequency of a desired signal in said band of frequencies, and another of said circuits being resonant below the desired signal frequency, but only enough below to permit it to substantially increase the voltage gain in the coupling circuit.

12. In a coupling circuit for a signal translating device, the combination including, a tunable primary circuit, and a tunable secondary circuit solely magnetically coupled to said primary circuit, both of said tunable circuits being resonant to different frequencies.

13. A coupling network, adapted for use between a source of radio frequency signal energy and an amplifier tube, comprising a coil connected to the source, adjustable means for resonating the coil to any desired frequency in a range of from 500 to 1500 kilocycles, a second coil connected to the input electrodes of the tubes, adjustable means for resonating the second coil through the said range, an untuned reactance coupling said coils, means for simultaneously varying both said adjustable means, said first adjustable means being so related to the second adjustable means that a predetermined frequency difference is maintained between the resonant frequencies of said coils at all times.

14. A coupling network, adapted for use between a source of radio frequency signal energy and an amplifier tube, comprising a coil connected to the source, adjustable means for resonating the coil to any desired frequency in a range of from 500 to 1500 kilocycles, a second coil connected to the input electrodes of the tubes, adjustable means for resonating the second coil through the said range, an untuned reactance coupling said coils, means for simultaneously varying both said adjustable means, said first adjustable means being so related to the second adjustable means that a. predetermined frequency difference is maintained between the resonant frequencies of said coils at all times, said difference being sufficient to produce a fiat gain characteristic over said frequency range.

HENRY C. FORBES. 

